1. Field of the Invention
This invention relates generally to rotary actuators for disk drives and in particular to pivot bearings which allow rotation through a limited angle.
2. Description of Prior Art
It is well known to employ rotary actuators for positioning magnetic transducers (heads) of a magnetic disk memory medium over selected information bearing tracks on the disk(s). British Patent 1,342,495 issued to D. E. Cuzner et al. on Jan. 3, 1974 and titled "Transducer arms for magnetic disc recording and/or reproducing apparatus" first teaches such an actuator for a single disk. U.S. Pat. No. 3,849,800 also teaches the same. This actuator arm is claimed as being angularly moveable and described as being rotatably mounted on a shaft by a bearing. The application of a rotary actuator to multiple disks is first described in the IBM Technical Disclosure Bulletin Vol. 16 No. 10, March 1974, by D. E. Cuzner et al. This teaches a single rotating arm comprising multiple legs which interleave a stack of disks. The arm is described as being pivoted on a shaft carried by bearings which are housed in a clamping structure. Because the movement of the transducers has to be controlled with great precision by a servo system, it is necessary that the actuator should have a small frictional resistance to rotation, while at the same time have no looseness or backlash due to clearances in the pivot bearings. For the same reason it is also necessary that the arm structure, including the pivot, should be free of resonant vibrational modes which could effect the stability of the servo system. An article by J.S. Heath in the IBM Journal of Research and Development (July 1976, pages 389 to 397) describes the basic operation of the rotary actuator and the requirements of the servo system to ensure satisfactory operation. This article describes a pivot system using needle roller bearings with spring loaded inserts to remove radial clearance. Such a bearing system is only possible because in disk drive rotary actuators the total angular range of movement necessary to move the recording transducers across all the tracks is limited to about thirty degrees.
The great majority of rotary actuators that have been produced however employ normal deep groove or angular contact ball bearings to form the pivot. These standard bearings are freely available and can be arranged to provide most of the necessary mechanical properties. In order to remove internal clearance it is necessary to use a pair of bearings assembled on a single shaft. The bearings are positioned so that each exerts a small axial force on the other. This force eliminates the internal clearances of the bearings. The force has to be adjusted carefully to provide adequate dynamic properties yet without increasing the frictional resistance to rotation (torque) of the assembly to an unacceptable extent. This frictional torque arises from the fact that there are many points of contact in a pair of ball bearings, typically between 24 and 32 depending on the number of balls. There is also a large ratio between the rotational velocity of the balls and that of the arm rotation. The net effect is that the frictional forces between the balls and the surfaces of the races on which they roll, which are naturally very small indeed, are greatly amplified and add together to produce a significant frictional resistance to rotation of the assembly. It is a characteristic of this friction that it is very variable as it depends upon the history of prior rotational movement of the pivot. A word that describes this form of frictional behavior is "hysteretic."
It is possible to design a servo to mitigate some of the effects of frictional influences on the motion and positional accuracy of the controlled element, the recording head. Methods are described in U.S. Pat. No. 4,536,809 issued on Aug. 20, 1985 to M. Sidman and entitled "Adaptive Misposition Correcting Method and Apparatus for Magnetic Disk Servo Systems" and also in an article by M. D. Sidman entitled "Control Systems Technology in Digital's Disk Drives" published in the Digital Technical Journal No. 8, February 1989. In this article the unwanted torque is referred to as bias. It is explained that the friction forces must be stable and predictable for this adaptive control method to be effective. This is not the case with the friction that is experienced in the ball bearing pivot systems normally employed in actuators. It will be appreciated that in order to increase the total information that can be stored in a disk drive, it is beneficial to increase the density of storage of the data on the disks, and that one way to accomplish this is to space the information tracks more closely. To do this demands that the accuracy of the mechanism and servo have to be improved. So for the reasons explained above, the ball bearing pivot places limitations on the density of the recorded tracks in a storage device.
The use of a pair of ball bearings as a disk drive pivot also limits the dynamic characteristics of the actuator mechanism. This is because of the demands of accuracy that are made by ball bearings. Firstly the outer race rings of the bearings themselves are relatively massive as they are made of steel and require to be strong enough to maintain a very high roundness accuracy. Then the housings into which the races are installed are similarly massive both for the same reasons and also to provide the dimensional control needed to maintain the small pre-load between the bearings that has been described previously. Thus actuator arm structures known previously have relatively high mass in the central structures proximate to the pivot. This mass coupled with the stiffness of the pivot produces a resonant mode that can limit the gain of the servo which, as explained previously, can limit the minimum spacing between the recorded tracks.
In addition to this disadvantageous aspect of the mass of the pivot, this same mass also contributes a small extent to the rotational inertia of the arm which determines how fast the recording transducer can be moved to new data. This is a part of the data access time of the disk drive, so it can be seen that a further disadvantage of the ball bearing pivot is that it slows the speed of performance of the disk drive.
In order to achieve the low friction torque characteristics which has previously been explained is necessary for satisfactory operation, it has been found important to use bearings of rather high quality in respect of surface finish, race accuracy, and cleanliness of the lubricant. To ensure that adequately low friction is maintained after assembly of the pivot, the alignment of the inner and outer raceways of the two bearings must also be accurately maintained which requires that the shaft and outer bearing housing must be precision machined, and assembled under careful control. So the pivot arrangement known previously is relatively difficult to manufacture by mass production processes, and is therefore expensive.
It will be appreciated that the disadvantages described in the preceding paragraphs are linked to the use of a general purpose bearing technology which has been improved over a long period of time for a vast range of other applications all of which involve continuous rotation or relatively high specific loading or both. Yet neither of these characteristics is needed in the actuator pivot application. The functional requirements for a disk drive actuator pivot are only that it should be rigid in all degrees of freedom other than rotation, and have low or predictable friction. It is not even essential that the rotation is strictly about a fixed point.
A further important part of a disk drive actuator is the flexible electrical connection that is required between the rotating actuator and the base, in order to provide an electrical connection to the transducer heads. A typical form for this connection is as described in U.S. Pat. No. 4,933,785 issued Jun. 12, 1990 to J. H. Morehouse and titled "Disk Drive Apparatus Using Dynamic Loading/Unloading." This describes a loop of flexible "printed" circuit cable material connected to the actuator arm at one end and to a fixed support at the other. The actuator connection is chosen to be as near to the center of rotation of the pivot as is possible. This is in order to minimize the twisting moment that the loop can exert on the arm. The main purpose of reducing this twisting or biasing moment is to thereby reduce the extent to which the head is forced away from the recorded track. A secondary purpose is to minimize the power dissipated by the servo in resisting this moment. The ball bearing pivot, including as it does a shaft and outer race housing for the reasons previously described, has necessitated that the attachment of the flexible connection to the actuator cannot be very close to the center of rotation in known rotary actuators. A specific geometrical arrangement to reduce the twisting moment applied to the actuator is described in the above referenced patent. However, the patent still teaches a very long flexible cable. This illustrates that even this improved geometrical arrangement is only a partial solution to reducing the bias torque to an acceptable level.
It will be appreciated that the length required of this part increases its cost as it is manufactured from expensive materials and by an expensive process.